1. Field of the Invention
This invention relates to a light scanning apparatus used in a laser exposure device for drawing an accurate pattern with a laser beam, and more particularly to a light scanning apparatus having a function for correcting the tilt of a surface of a scan deflector for scanning a light beam.
2. Description of the Prior Art
An ideal polygon mirror, such as a scan deflector, reflecting surface which is parallel with the rotational axis, and spot scans the same line when a laser beam is reflected by any surface. In practice, however, an inclination of the reflecting surface occurs with respect to the rotational axis owing to machining errors, etc. In other words, the tilt of a surface takes place. Consequently, the scanning line for each reflecting surface is displaced in an auxiliary scanning direction. In this specification, the direction in which scanning is performed on the scanning surface is referred to as the principal scanning direction, and the direction that is normal to the principle scanning direction is referred to as the auxiliary scanning direction.
It is technically difficult to remove the tilt surface error of by improving the machining accuracy of the polygon mirror which often involves significant costs.
In view of the above, there has heretofore been proposed means for removing the adverse affection of this surface tilting by providing a correcting optical system.
In Japanese Patent Early Laid-open Publication No. Sho 59-15216, there is disclosed a method for diminishing the adverse affection due to the surface tilting of the reflecting surface by converging light beams in such a manner as to form a line image in the principal scanning direction on a reflecting surface of a polygon mirror.
However, it is still difficult to completely correct the surface tilting by this method. Also, when this method is adopted, it becomes necessary to provide an anamorphic lens system which has different powers in a principal scanning plane and in an auxiliary scanning plane in order to form a line image, and it is difficult to manufacture such a lens system, compared to a spherical lens. In particular, it is difficult to manufacture an anamorphic lens system that is suitable for an apparatus that is required to have an ability to draw a high accuracy pattern.
Such a construction as shown, for example, in Japanese Patent Early Laid-open Publication No. Sho 56-107212 discloses a case where the surface tilting is to be corrected while utilizing a spherical lens as a lens system, in that an acousto-optic modulator (AO modulator) is disposed between a light source and a polygon mirror in order to finely deform the angle of incidence in the auxiliary scanning direction.
Although it is not disclosed in this Publication, a beam expander system is actually provided in view of the necessity to make large the diameter of the light beam which is made incident to a scanning lens in order to reduce the spot on the scanning surface.
An AO modulator has a response frequency which is in inverse proportion to the diameter of the light beam on the modulator. Thus, the AO modulator must be disposed in a position having a small diameter of the light beam in order to render a modulation function. Therefore, the AO modulator is often disposed in parallel light beams on the light source side from the beam expander system.
FIG. 14 shows an example in which the AO modulator is disposed in the parallel light beams on the light source side (left-hand side in the Figures) from the beam expander system, FIG. 14 shows one example using a beam expander of a Galilean type. In the Figure, the solid line shows an optical path before being deflected by the AO modulator and the broken lines show the optical path after being deflected by the AO modulator.
In the Figure, if the focal length of a first lens group L.sub.1 is taken as f.sub.1 (&lt;0) and the focal length of a second lens group L.sub.2 is taken as f.sub.2 (&gt;0), the magnification M of the beam expander system can be expressed as follow; EQU M=-f.sub.2 /f.sub.1
and if the angle of inclination of the light beam for correcting the surface tilting is taken as .theta..sub.P, the angle of deflection .theta..sub.m is expressed by the following relation 1. EQU .theta..sub.m =M.multidot..theta..sub.P 1
Next, if the distance from the AO modulator to the first lens group L.sub.1 is taken as d.sub.0 and the distance from the second lens group L.sub.2 to the polygon mirror is taken as d.sub.1, the displaced amount S of the light beam can be expressed by the following relation 2. EQU S={M.sup.2 .multidot.d.sub.0 +M(f.sub.1 +f.sub.2)+d.sub.1 }.multidot..theta..sub.P 2
For example, let us suppose there is a beam expander system for expanding the diameter of a light beam from 1 mm to 20 mm where f.sub.1 =10, f.sub.2 =200, M=-20, and d.sub.0 =100, and the surface tilting of the reflecting surface of the polygon mirror is taken as 10", and the correction angle .theta..sub.P becomes 20". Therefore, the angle of deflection of the AO modulator becomes .theta..sub.m =-0.11.degree. from the relation 1. Also, the displaced amount of the light beam on the reflecting surface of the polygon mirror becomes S=3.5 from the relation 2, assuming d.sub.1 =100.
Accordingly, although the angle error of the reflected light beam due to the surface tilting can be corrected, the beam on the reflecting surface of the polygon mirror shifts in a parallel relation in accordance with this correction. In addition the position of the beam, which is incident to the scanning lens, gets out of a regular position. Accordingly a sharpness of the spot on the scanning surface deteriorates. This gives rise to a problem when a high accuracy drawing is to be obtained.
Also, there is another problem in that the controlling of the AO modulator is difficult in order to obtain a fine angle such as .theta..sub.m =-0.11.degree..
Furthermore, in the Japanese Patent Early Laid-open Publication No. SHO 62-235918, there is disclosed a construction in which two modulators are provided, one for correcting the surface tilting and the other for rectifying the shifting amount of the light beam. With this construction, however, in order to provide two AO modulators, the degree of freedom for designing the optical system becomes small and the electrical controlling becomes complicated.